CATALOGUE OF GEMS. 498 



to their original beauty, thej^ are liy no means to be relied upon, so 

 that care should be taken to preserve tine pearls by carefully wiping 

 them after use with a clean, soft cloth and keeping them wrapped in 

 a clean cloth in a closed box. 



Although nearh' all bivalves with nacreous shells occasionally yield 

 pearls, practically all of the pearls of commerce are obtained from a 

 few families only of moliusks, prominent among which are the Avicu- 

 lidae, Unionidae, and Mytilidae. The pearl oyster of the Pacitic and 

 Indian oceans, which has j'ielded the bulk of the pearls of the world, 

 belongs to the tirst of these groups. The unio, or fresh-water mussel, 

 so abundant in the rivers and lakes of North America, belongs to the 

 second. 



Coral is essentially carbonate of lime produced \ij gelatinous marine 

 animals called poh'ps. The coral-forming animals are very often, 

 though wrongly, called coral insects, and the coral is not built up as 

 bees build a comb, but grows, as do the bones of other animals, being 

 produced or secreted b}^ a peculiar layer of the skin. The small star- 

 like pits on a branch of coral represent the places where the various 

 members of the colon}^ once grew. 



Although coral is found abundantly in the seas of many parts of the 

 globe, that adapted to purposes of ornament comes almost entirelj^ 

 from the African coast of the Mediterranean. The beds lie at a depth 

 of 500 to 800 feet below the surface of the sea, and the coral is won 

 by means of iron drags and nets. The coral varies in color from deep 

 red through pink to green, brown, yellow, white, and black. The red 

 coral, once the most valua])le, is now worth far less than the rose-pink, 

 while the white, yellow, and black corals are of very little value. 



Coral is frequently imitated in horn, bone, and ivory; also b}^ a 

 composition made up of plaster of paris, gum, and coloring matter. 



Marhles consist essentially of carbonate of lime, with more or less 

 carbonate of magnesia. They are fine to :oarse granular in structure, 

 and exhibit various colors, as white, yellow, red, green, blue, etc., 

 often clouded and giving a handsome eflect when polished. Statuary 

 marhle is pure white and fine grained; the best is from Paros and 

 Carrara, Italy. Ar^chitectural marlle is both white and colored. The 

 Cljjolin is white, tinged with green. The Siena is yellow, veined or 

 clouded with brownish red. The Mandelato is light red with white 

 spots. The B<(rdi(/lio is gray with dark cloudings. Verde- Antique is 

 clouded yellowish to bluish green. The Partor or Egyptian warUe is 

 black, veined with j^ellow. Lumachelle ox fire marble is a dark brown 

 shell marble with brilliant fire-like or chatoyant internal reflections. 

 Madreporlc rnarhle contains corals. Huin niarhle is a kind showing, 

 when polished, figures bearing a resemblance to cities, castles, etc. , in 

 ruins. OoUtr is a concretionar}' massive limestone made up of minute 

 spherical grains resembling the roe of a fish, the name coming from 



